Two-piece golf ball

ABSTRACT

A golf ball  2  includes a core  4  and a cover  6 . A value V calculated by the following mathematical formula is equal to or less than 1080 Hz. 
         V =NF(2)2−2/3*NF(2)1
 
     In the mathematical formula, NF(2)1 represents a secondary natural frequency of the core  4 , and NF(2)2 represents a secondary natural frequency of the golf ball  2 . A difference (H1s−H1o) between a Shore C hardness H1s at a surface of the core  4  and a Shore C hardness H1o at a central point of the core  4  is equal to or greater than 10. An amount of compressive deformation Df1 of the core  4  is equal to or greater than 4.1 mm.

This application claims priority on Patent Application No. 2015-242909filed in JAPAN on Dec. 14, 2015. The entire contents of this JapanesePatent Application are hereby incorporated by reference.

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention relates to golf balls. Specifically, the presentinvention relates to so-called two-piece golf balls including a core anda cover.

Description of the Related Art

In golf, golf balls are hit with a wood type club, an iron type club, ahybrid type club (utility), and a putter, etc. Feel at impact uponhitting is of interest to golf players. In general, golf players desiregolf balls having soft feel at impact.

In play by beginners, the frequency of a mishit is high. Therefore,beginners are insensitive to feel at impact when hitting a golf ballwith a wood type club, an iron type club, or a hybrid type club.

Meanwhile, in putting, even beginners often hit golf balls at the sweetspots of putters. Even beginners are sensitive to feel at impact uponputting. Beginners desire golf balls with which soft feel at impact isobtained upon putting.

So-called thread-wound balls used to be mainstream golf balls. Atpresent, thread-wound balls are almost not available commercially. Inrecent golf, two-piece balls, three-piece balls, four-piece balls,five-piece balls, six-piece balls, and the like are used.

A two-piece ball includes a core and a cover. The structure of thetwo-piece ball is simple. The two-piece ball can be manufactured at lowcost. Proposals concerning two-piece balls are disclosed in JPH11-76461(U.S. Pat. No. 6,123,629) and U.S. Pat. No. 5,971,870.

Upon shots by beginners, golf balls often fly in an unintendeddirection. Golf balls often fall into a pond or fly into woods.Beginners often loose golf balls. Therefore, beginners do not preferexpensive golf balls. Two-piece balls are suitable for beginners, sincetwo-piece balls can be manufactured at low cost. As descried above,beginners prefer soft feel at impact upon putting. Improvement of feelat impact of two-piece balls upon putting is desired.

An object of the present invention is to provide a two-piece golf ballhaving excellent feel at impact upon putting.

SUMMARY OF THE INVENTION

A two-piece golf ball according to the present invention includes a coreand a cover positioned outside the core. In the golf ball, a value Vcalculated by the following mathematical formula is equal to or lessthan 1080 Hz.

V=NF(2)2−2/3*NF(2)1

In the mathematical formula, NF(2)1 represents a secondary naturalfrequency of the core, and NF(2)2 represents a secondary naturalfrequency of the golf ball.

The two-piece golf ball according to the present invention has a simplestructure. The golf ball can be manufactured at low cost. Since thevalue V is equal to or less than 1080 Hz, the feel at impact of the golfball upon putting is soft. The golf ball can achieve both a low priceand desired feel at impact.

Preferably, a difference (H1s−H1o) between a Shore C hardness H1s at asurface of the core and a Shore C hardness H1o at a central point of thecore is equal to or greater than 10.

Preferably, the value V is equal to or less than 1040 Hz. Furtherpreferably, the value V is equal to or less than 1000 Hz.

Preferably, an amount of compressive deformation Df1 of the core isequal to or greater than 4.1 mm.

Preferably, an amount of compressive deformation Df2 of the two-piecegolf ball is equal to or greater than 3.5 mm.

Preferably, a thickness T2 of the cover is equal to or greater than 0.80mm but equal to or less than 2.00 mm. Preferably, a Shore D hardness H2of the cover is equal to or greater than 50 but equal to or less than65.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of a golf ball according to oneembodiment of the present invention;

FIG. 2 is a conceptual diagram showing a device for measuring a naturalfrequency of the golf ball in FIG. 1; and

FIG. 3 is a graph showing a relationship between a secondary naturalfrequency NF(2)1 of a core and a secondary natural frequency NF(2)2 ofthe golf ball.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The following will describe in detail the present invention based onpreferred embodiments with appropriate reference to the drawings.

A golf ball 2 shown in FIG. 1 includes a spherical core 4 and a cover 6positioned outside the core 4. In the present embodiment, the cover 6 isjoined directly to the core 4. The golf ball 2 is a so-called two-pieceball. The golf ball 2 has a plurality of dimples 8 on the surfacethereof. Of the surface of the golf ball 2, a part other than thedimples 8 is a land 10. The golf ball 2 includes a paint layer and amark layer on the external side of the cover 6 although these layers arenot shown in the drawing.

The golf ball 2 preferably has a diameter of equal to or greater than 40mm but equal to or less than 45 mm. From the standpoint of conformity tothe rules established by the United States Golf Association (USGA), thediameter is particularly preferably equal to or greater than 42.67 mm.In light of suppression of air resistance, the diameter is morepreferably equal to or less than 44 mm and particularly preferably equalto or less than 42.80 mm. The golf ball 2 preferably has a weight ofequal to or greater than 40 g but equal to or less than 50 g. In lightof attainment of great inertia, the weight is more preferably equal toor greater than 44 g and particularly preferably equal to or greaterthan 45.00 g. From the standpoint of conformity to the rules establishedby the USGA, the weight is particularly preferably equal to or less than45.93 g.

The core 4 is formed by crosslinking a rubber composition. Examples ofpreferable base rubbers for use in the rubber composition includepolybutadienes, polyisoprenes, styrene-butadiene copolymers,ethylene-propylene-diene copolymers, and natural rubbers. In light ofresilience performance, polybutadienes are preferable. When apolybutadiene and another rubber are used in combination, it ispreferred if the polybutadiene is a principal component. Specifically,the proportion of the polybutadiene to the entire base rubber ispreferably equal to or greater than 50% by weight and particularlypreferably equal to or greater than 80% by weight. A polybutadiene inwhich the proportion of cis-1,4 bonds is equal to or greater than 80% isparticularly preferable.

The rubber composition of the core 4 preferably includes aco-crosslinking agent. Preferable co-crosslinking agents in light ofresilience performance of the golf ball 2 are monovalent or bivalentmetal salts of an α,β-unsaturated carboxylic acid having 2 to 8 carbonatoms. Examples of preferable co-crosslinking agents include zincacrylate, magnesium acrylate, zinc methacrylate, and magnesiummethacrylate. In light of resilience performance of the golf ball 2,zinc acrylate and zinc methacrylate are particularly preferable.

The rubber composition may include a metal oxide and an α,β-unsaturatedcarboxylic acid having 2 to 8 carbon atoms. They both react with eachother in the rubber composition to obtain a salt. The salt serves as aco-crosslinking agent. Examples of preferable α,β-unsaturated carboxylicacids include acrylic acid and methacrylic acid. Examples of preferablemetal oxides include zinc oxide and magnesium oxide.

The amount of the co-crosslinking agent per 100 parts by weight of thebase rubber is preferably equal to or greater than 10 parts by weight.The golf ball 2 that includes the core 4 in which this amount is equalto or greater than 10 parts by weight has excellent resilienceperformance. In this respect, this amount is more preferably equal to orgreater than 15 parts by weight and particularly preferably equal to orgreater than 20 parts by weight.

The amount of the co-crosslinking agent per 100 parts by weight of thebase rubber is preferably equal to or less than 40 parts by weight. Thegolf ball 2 that includes the core 4 in which this amount is equal to orless than 40 parts by weight has soft feel at impact upon putting. Inthis respect, this amount is more preferably equal to or less than 35parts by weight and particularly preferably equal to or less than 30parts by weight.

Preferably, the rubber composition of the core 4 includes an organicperoxide. The organic peroxide serves as a crosslinking initiator. Theorganic peroxide contributes to the resilience performance of the golfball 2. Examples of suitable organic peroxides include dicumyl peroxide,1,1-bis(t-butylperoxy)-3,3,5-trimethylcyclohexane,2,5-dimethyl-2,5-di(t-butylperoxy)hexane, and di-t-butyl peroxide. Anorganic peroxide with particularly high versatility is dicumyl peroxide.

The amount of the organic peroxide per 100 parts by weight of the baserubber is preferably equal to or greater than 0.1 parts by weight. Thegolf ball 2 that includes the core 4 in which this amount is equal to orgreater than 0.1 parts by weight has excellent resilience performance.In this respect, this amount is more preferably equal to or greater than0.3 parts by weight and particularly preferably equal to or greater than0.5 parts by weight.

The amount of the organic peroxide per 100 parts by weight of the baserubber is preferably equal to or less than 3.0 parts by weight. The golfball 2 that includes the core 4 in which this amount is equal to or lessthan 3.0 parts by weight has soft feel at impact upon putting. In thisrespect, this amount is more preferably equal to or less than 2.5 partsby weight and particularly preferably equal to or less than 2.0 parts byweight.

The rubber composition of the core 4 includes an organic sulfurcompound. Organic sulfur compounds include naphthalenethiol compounds,benzenethiol compounds, and disulfide compounds.

Examples of naphthalenethiol compounds include 1-naphthalenethiol,2-naphthalenethiol, 4-chloro-1-naphthalenethiol,4-bromo-1-naphthalenethiol, 1-chloro-2-naphthalenethiol,1-bromo-2-naphthalenethiol, 1-fluoro-2-naphthalenethiol,1-cyano-2-naphthalenethiol, and 1-acetyl-2-naphthalenethiol.

Examples of benzenethiol compounds include benzenethiol,4-chlorobenzenethiol, 3-chlorobenzenethiol, 4-bromobenzenethiol,3-bromobenzenethiol, 4-fluorobenzenethiol, 4-iodobenzenethiol,2,5-dichlorobenzenethiol, 3,5-dichlorobenzenethiol,2,6-dichlorobenzenethiol, 2,5-dibromobenzenethiol,3,5-dibromobenzenethiol, 2-chloro-5-bromobenzenethiol,2,4,6-trichlorobenzenethiol, 2,3,4,5,6-pentachlorobenzenethiol,2,3,4,5,6-pentafluorobenzenethiol, 4-cyanobenzenethiol,2-cyanobenzenethiol, 4-nitrobenzenethiol, and 2-nitrobenzenethiol.

Examples of disulfide compounds include diphenyl disulfide,bis(4-chlorophenyl)disulfide, bis(3-chlorophenyl)disulfide,bis(4-bromophenyl)disulfide, bis(3-bromophenyl)disulfide,bis(4-fluorophenyl)disulfide, bis(4-iodophenyl)disulfide,bis(4-cyanophenyl)disulfide, bis(2,5-dichlorophenyl)disulfide,bis(3,5-dichlorophenyl)disulfide, bis(2,6-dichlorophenyl)disulfide,bis(2,5-dibromophenyl)disulfide, bis(3,5-dibromophenyl)disulfide,bis(2-chloro-5-bromophenyl)disulfide,bis(2-cyano-5-bromophenyl)disulfide,bis(2,4,6-trichlorophenyl)disulfide,bis(2-cyano-4-chloro-6-bromophenyl)disulfide,bis(2,3,5,6-tetrachlorophenyl)disulfide,bis(2,3,4,5,6-pentachlorophenyl)disulfide, andbis(2,3,4,5,6-pentabromophenyl)disulfide.

In light of resilience performance of the golf ball 2, the amount of theorganic sulfur compound per 100 parts by weight of the base rubber ispreferably equal to or greater than 0.1 parts by weight and particularlypreferably equal to or greater than 0.2 parts by weight. In light offeel at impact upon putting, the amount is preferably equal to or lessthan 1.5 parts by weight, more preferably equal to or less than 1.0parts by weight, and particularly preferably equal to or less than 0.8parts by weight. Two or more organic sulfur compounds may be used incombination.

The rubber composition of the core 4 may include a filler for thepurpose of specific gravity adjustment and the like. Examples ofsuitable fillers include zinc oxide, barium sulfate, calcium carbonate,and magnesium carbonate. The amount of the filler is determined asappropriate so that the intended specific gravity of the core 4 isaccomplished.

The rubber composition may include various additives, such as sulfur, acarboxylic acid, a carboxylate, an anti-aging agent, a coloring agent, aplasticizer, a dispersant, and the like, in an adequate amount. Therubber composition may include crosslinked rubber powder or syntheticresin powder.

The core 4 preferably has a diameter of equal to or greater than 39.0mm. In the golf ball 2 that includes the core 4 having a diameter ofequal to or greater than 39.0 mm, the cover 6 is thin. Therefore, withthe golf ball 2, the feel at impact upon putting is soft. Furthermore,the golf ball 2 has excellent resilience performance. In these respects,the diameter is more preferably equal to or greater than 39.3 mm andparticularly preferably equal to or greater than 39.5 mm. In light ofdurability of the golf ball 2, the diameter is preferably equal to orless than 41.0 mm, more preferably equal to or less than 40.6 mm, andparticularly preferably equal to or less than 40.2 mm.

The core 4 preferably has an amount of compressive deformation Df1 ofequal to or greater than 4.1 mm. With the core 4 having an amount ofcompressive deformation Df1 of equal to or greater than 4.1 mm, the feelat impact upon putting is soft. In this respect, the amount ofcompressive deformation Df1 is more preferably equal to or greater than4.2 mm and particularly preferably equal to or greater than 4.4 mm. Inlight of resilience performance of the golf ball 2, the amount ofcompressive deformation Df1 is preferably equal to or less than 6.5 mm,more preferably equal to or less than 6.0 mm, and particularlypreferably equal to or less than 5.5 mm.

For measurement of the amount of compressive deformation, a YAMADA typecompression tester is used. In the tester, a sphere (the core 4 or thegolf ball 2) is placed on a hard plate made of metal. Next, a cylindermade of metal gradually descends toward the sphere. The sphere, squeezedbetween the bottom face of the cylinder and the hard plate, becomesdeformed. A migration distance of the cylinder, starting from the statein which an initial load of 98 N is applied to the sphere up to thestate in which a final load of 1274 N is applied thereto, is measured. Amoving speed of the cylinder until the initial load is applied is 0.83mm/s. A moving speed of the cylinder after the initial load is applieduntil the final load is applied is 1.67 mm/s.

The difference (H1s−H1o) between a Shore C hardness H1s at the surfaceof the core 4 and a Shore C hardness H1o at the central point of thecore 4 is preferably equal to or greater than 10. The core 4 having adifference (H1s−H1o) of equal to or greater than 10 has a so-calledouter-hard/inner-soft structure. When the golf ball 2 including the core4 is hit with a driver, the spin is suppressed. When the golf ball 2including the core 4 is hit with a driver, a high launch angle isobtained.

Upon a shot with a driver, an appropriate trajectory height andappropriate flight duration are required. With the golf ball 2 thatachieves a desired trajectory height and desired flight duration at ahigh spin rate, the run after landing is short. With the golf ball 2that achieves a desired trajectory height and desired flight duration ata high launch angle, the run after landing is long. In light of flightdistance, the golf ball 2 that achieves a desired trajectory height anddesired flight duration at a high launch angle is preferable. The core 4having an outer-hard/inner-soft structure can contribute to a highlaunch angle and a low spin rate as described above. Although the amountof compressive deformation Df1 is small, the core 4 can contribute tothe flight performance of the golf ball 2.

In light of flight performance, the difference (H1s−H1o) is morepreferably equal to or greater than 11 and particularly preferably equalto or greater than 12. In light of durability of the golf ball 2, thedifference (H1s−H1o) is preferably equal to or less than 30, morepreferably equal to or less than 28, and particularly preferably equalto or less than 25.

In light of durability and resilience performance, the central hardnessH1o is preferably equal to or greater than 40, more preferably equal toor greater than 45, and particularly preferably equal to or greater than50. In light of spin suppression, the hardness H1o is preferably equalto or less than 70, more preferably equal to or less than 65, andparticularly preferably equal to or less than 60.

The hardness H1o is measured with a Shore C type hardness scale mountedto an automated hardness meter (trade name “digi test II” manufacturedby Heinrich Bareiss Prüfgerätebau GmbH). The hardness scale is pressedagainst the central point of the cross-section of a hemisphere obtainedby cutting the golf ball 2. The measurement is conducted in theenvironment of 23° C.

In light of spin suppression, the surface hardness H1s is preferablyequal to or greater than 64, more preferably equal to or greater than66, and particularly preferably equal to or greater than 68. In light ofdurability of the golf ball 2, the hardness H1s is preferably equal toor less than 85, more preferably equal to or less than 83, andparticularly preferably equal to or less than 80.

The hardness H1s is measured with a Shore C type hardness scale mountedto an automated hardness meter (trade name “digi test II” manufacturedby Heinrich Bareiss Prüfgerätebau GmbH). The hardness scale is pressedagainst the surface of the core 4. The measurement is conducted in theenvironment of 23° C.

The core 4 preferably has a weight of equal to or greater than 10 g butequal to or less than 42 g. The temperature for crosslinking the core 4is equal to or higher than 140° C. but equal to or lower than 180° C.The time period for crosslinking the core 4 is equal to or longer than10 minutes but equal to or shorter than 60 minutes.

The cover 6 is positioned outside the core 4. The cover 6 is theoutermost layer except the mark layer and the paint layer. The cover 6is formed from a thermoplastic resin composition. Examples of the basepolymer of the resin composition include ionomer resins, thermoplasticpolyester elastomers, thermoplastic polyamide elastomers, thermoplasticpolyurethane elastomers, thermoplastic polyolefin elastomers, andthermoplastic polystyrene elastomers. Ionomer resins are particularlypreferable. Ionomer resins are highly elastic. The golf ball 2 thatincludes the cover 6 including an ionomer resin has excellent resilienceperformance. The cover 6 may be formed from a thermosetting resincomposition.

An ionomer resin and another resin may be used in combination. In thiscase, in light of resilience performance, the ionomer resin is includedas the principal component of the base polymer. The proportion of theionomer resin to the entire base polymer is preferably equal to orgreater than 50% by weight, more preferably equal to or greater than 70%by weight, and particularly preferably equal to or greater than 85% byweight.

Examples of preferable ionomer resins include binary copolymers formedwith an α-olefin and an α,β-unsaturated carboxylic acid having 3 to 8carbon atoms. A preferable binary copolymer includes 80% by weight ormore but 90% by weight or less of an α-olefin, and 10% by weight or morebut 20% by weight or less of an α,β-unsaturated carboxylic acid. Thebinary copolymer has excellent resilience performance. Examples of otherpreferable ionomer resins include ternary copolymers formed with: anα-olefin; an α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms;and an α,β-unsaturated carboxylate ester having 2 to 22 carbon atoms. Apreferable ternary copolymer includes 70% by weight or more but 85% byweight or less of an α-olefin, 5% by weight or more but 30% by weight orless of an α,β-unsaturated carboxylic acid, and 1% by weight or more but25% by weight or less of an α,β-unsaturated carboxylate ester. Theternary copolymer has excellent resilience performance. For the binarycopolymer and the ternary copolymer, preferable α-olefins are ethyleneand propylene, while preferable α,β-unsaturated carboxylic acids areacrylic acid and methacrylic acid. A particularly preferable ionomerresin is a copolymer formed with ethylene and acrylic acid. Anotherparticularly preferable ionomer resin is a copolymer formed withethylene and methacrylic acid.

In the binary copolymer and the ternary copolymer, some of the carboxylgroups are neutralized with metal ions. Examples of metal ions for usein neutralization include sodium ion, potassium ion, lithium ion, zincion, calcium ion, magnesium ion, aluminum ion, and neodymium ion. Theneutralization may be carried out with two or more types of metal ions.Particularly suitable metal ions in light of resilience performance anddurability of the golf ball 2 are sodium ion, zinc ion, lithium ion, andmagnesium ion.

Specific examples of ionomer resins include trade names “Himilan 1555”,“Himilan 1557”, “Himilan 1605”, “Himilan 1706”, “Himilan 1707”, “Himilan1856”, “Himilan 1855”, “Himilan AM7311”, “Himilan AM7315”, “HimilanAM7317”, “Himilan AM7329”, and “Himilan AM7337”, manufactured by DuPont-MITSUI POLYCHEMICALS Co., Ltd.; trade names “Surlyn 6120”, “Surlyn6910”, “Surlyn 7930”, “Surlyn 7940”, “Surlyn 8140”, “Surlyn 8150”,“Surlyn 8940”, “Surlyn 8945”, “Surlyn 9120”, “Surlyn 9150”, “Surlyn9910”, “Surlyn 9945”, “Surlyn AD8546”, “HPF1000”, and “HPF2000”,manufactured by E.I. du Pont de Nemours and Company; and trade names“IOTEK 7010”, “IOTEK 7030”, “IOTEK 7510”, “IOTEK 7520”, “IOTEK 8000”,and “IOTEK 8030”, manufactured by ExxonMobil Chemical Corporation. Twoor more ionomer resins may be used in combination.

The resin composition of the cover 6 may include a styreneblock-containing thermoplastic elastomer. The styrene block-containingthermoplastic elastomer includes a polystyrene block as a hard segment,and a soft segment. A typical soft segment is a diene block. Examples ofcompounds for the diene block include butadiene, isoprene,1,3-pentadiene, and 2,3-dimethyl-1,3-butadiene. Butadiene and isopreneare preferable. Two or more compounds may be used in combination.

Examples of styrene block-containing thermoplastic elastomers includestyrene-butadiene-styrene block copolymers (SBS),styrene-isoprene-styrene block copolymers (SIS),styrene-isoprene-butadiene-styrene block copolymers (SIBS), hydrogenatedSBS, hydrogenated SIS, and hydrogenated SIBS. Examples of hydrogenatedSBS include styrene-ethylene-butylene-styrene block copolymers (SEBS).Examples of hydrogenated SIS include styrene-ethylene-propylene-styreneblock copolymers (SEPS). Examples of hydrogenated SIBS includestyrene-ethylene-ethylene-propylene-styrene block copolymers (SEEPS).

In light of resilience performance of the golf ball 2, the content ofthe styrene component in the styrene block-containing thermoplasticelastomer is preferably equal to or greater than 10% by weight, morepreferably equal to or greater than 12% by weight, and particularlypreferably equal to or greater than 15% by weight. In light of feel atimpact of the golf ball 2, the content is preferably equal to or lessthan 50% by weight, more preferably equal to or less than 47% by weight,and particularly preferably equal to or less than 45% by weight.

In the present invention, styrene block-containing thermoplasticelastomers include an alloy of an olefin and one or more membersselected from the group consisting of SBS, SIS, SIBS, SEBS, SEPS, andSEEPS. The olefin component in the alloy is presumed to contribute toimprovement of compatibility with another base polymer. The alloy cancontribute to the resilience performance of the golf ball 2. An olefinhaving 2 to 10 carbon atoms is preferable. Examples of suitable olefinsinclude ethylene, propylene, butene, and pentene. Ethylene and propyleneare particularly preferable.

Specific examples of polymer alloys include trade names “RABALONT3221C”, “RABALON T3339C”, “RABALON SJ4400N”, “RABALON SJ5400N”,“RABALON SJ6400N”, “RABALON SJ7400N”, “RABALON SJ8400N”, “RABALONSJ9400N”, and “RABALON SR04”, manufactured by Mitsubishi ChemicalCorporation. Other specific examples of styrene block-containingthermoplastic elastomers include trade name “Epofriend A1010”manufactured by Daicel Chemical Industries, Ltd., and trade name “SEPTONHG-252” manufactured by Kuraray Co., Ltd.

In light of feel at impact upon putting, the proportion of the styreneblock-containing thermoplastic elastomer to the entire base polymer ispreferably equal to or greater than 2% by weight, more preferably equalto or greater than 4% by weight, and particularly preferably equal to orgreater than 6% by weight. In light of spin suppression, the proportionis preferably equal to or less than 30% by weight, more preferably equalto or less than 25% by weight, and particularly preferably equal to orless than 20% by weight.

The resin composition of the cover 6 may include a coloring agent, afiller, a dispersant, an antioxidant, an ultraviolet absorber, a lightstabilizer, a fluorescent material, a fluorescent brightener, and thelike in an adequate amount. When the hue of the golf ball 2 is white, atypical coloring agent is titanium dioxide.

The cover 6 preferably has a thickness T2 of equal to or less than 2.00mm. The cover 6 having a thickness T2 of equal to or less than 2.00 mmdoes not impair soft feel at impact upon putting. In this respect, thethickness T2 is more preferably equal to or less than 1.85 mm andparticularly preferably equal to or less than 1.70 mm. In light of easeof forming the cover 6 and in light of durability of the golf ball 2,the thickness T2 is preferably equal to or greater than 0.80 mm, morepreferably equal to or greater than 0.95 mm, and particularly preferablyequal to or greater than 1.05 mm. The thickness T2 is measured at aposition immediately below the land 10.

From the standpoint that the golf ball 2 can have anouter-hard/inner-soft structure as a whole, the cover 6 has a Shore Dhardness H2 of preferably equal to or greater than 50, more preferablyequal to or greater than 53, and particularly preferably equal to orgreater than 55. In light of feel at impact upon putting, the hardnessH2 is preferably equal to or less than 65, more preferably equal to orless than 63, and particularly preferably equal to or less than 61.

The hardness H2 of the cover 6 is measured according to the standards of“ASTM-D 2240-68”. The hardness H2 is measured with a Shore D typehardness scale mounted to an automated hardness meter (trade name “digitest II” manufactured by Heinrich Bareiss Prüfgerätebau GmbH). For themeasurement, a sheet that is formed by hot press, is formed from thesame material as that of the cover 6, and has a thickness of about 2 mmis used. Prior to the measurement, a sheet is kept at 23° C. for twoweeks. At the measurement, three sheets are stacked.

The golf ball 2 preferably has an amount of compressive deformation Df2of equal to or greater than 3.5 mm. With the golf ball 2 having anamount of compressive deformation Df2 of equal to or greater than 3.5mm, the feel at impact upon putting is soft. In this respect, the amountof compressive deformation Df2 is more preferably equal to or greaterthan 3.6 mm and particularly preferably equal to or greater than 3.8 mm.In light of resilience performance of the golf ball 2, the amount ofcompressive deformation Df2 is preferably equal to or less than 6.0 mm,more preferably equal to or less than 5.5 mm, and particularlypreferably equal to or less than 5.0 mm.

FIG. 2 shows a device for measuring natural frequencies of the core 4and the golf ball 2. The device includes a vibration exciter 12, a plate14, a first acceleration pickup 16, and a second acceleration pickup 18.The plate 14 is mounted on the vibration exciter 12. A sphere (the core4 or the golf ball 2) is placed on the plate 14. The first accelerationpickup 16 is mounted on the plate 14. The second acceleration pickup 18is mounted on the sphere. Vibration is applied to the sphere by thevibration exciter 12. A signal of acceleration applied to the sphere isoutputted from the first acceleration pickup 16. A signal of theacceleration of the sphere is outputted from the second accelerationpickup 18. These signals are inputted into a dynamic signal analyzer. Bycalculation of the analyzer, a curve is obtained which shows arelationship between frequency and mechanical impedance at the sphere.The frequency at a minimum point of the curve is a natural frequency.The frequency at a minimum point that appears first on the curve is aprimary natural frequency. The frequency at a minimum point that appearssecond on the curve is a secondary natural frequency. The vibrationexciter 12 is typically trade name “PET”, manufactured by IMVCorporation. The dynamic signal analyzer is typically trade name“HP-5420A”, manufactured by Yokokawa Hewlett-Packard, Ltd.

As a result of thorough research, the present inventors have found thatsoft feel at impact upon putting is achieved when a secondary naturalfrequency NF(2)1 of the core 4 and a secondary natural frequency NF(2)2of the golf ball 2 have a predetermined relationship.

FIG. 3 is a graph showing a relationship between the secondary naturalfrequency NF(2)1 of the core 4 and the secondary natural frequencyNF(2)2 of the golf ball 2. In this graph, the horizontal axis indicatesthe secondary natural frequency NF(2)1 of the core, and the verticalaxis indicates the secondary natural frequency NF(2)2 of the golf ball.A straight line indicated by reference sign L1 in this graph isrepresented by the following mathematical formula.

NF(2)2=2/3*NF(2)1+1080

In the zone below the straight line L1 in this graph, a value Vcalculated by the following mathematical formula is equal to or lessthan 1080 Hz.

V=NF(2)2−2/3*NF(2)1

According to the finding by the present inventors, the golf ball 2having a value V of equal to or less than 1080 Hz has excellent feel atimpact upon putting. With the golf ball 2, the feel at impact uponputting is soft.

A straight line indicated by reference sign L2 in the graph of FIG. 3 isrepresented by the following mathematical formula.

NF(2)2=2/3*NF(2)1+1040

In the zone below the straight line L2 in this graph, the value V isequal to or less than 1040 Hz. The golf ball 2 having a value V of equalto or less than 1040 Hz has excellent feel at impact upon putting. Withthe golf ball 2, the feel at impact upon putting is soft.

A straight line indicated by reference sign L3 in the graph of FIG. 3 isrepresented by the following mathematical formula.

NF(2)2=2/3*NF(2)1+1000

In the zone below the straight line L3 in this graph, the value V isequal to or less than 1000 Hz. The golf ball 2 having a value V of equalto or less than 1000 Hz has excellent feel at impact upon putting. Withthe golf ball 2, the feel at impact upon putting is soft.

In light of durability and resilience performance of the golf ball 2,the value V is preferably equal to or greater than 800 Hz.

EXAMPLES Example 1

A rubber composition A was obtained by kneading 100 parts by weight of ahigh-cis polybutadiene (trade name “BR-730”, manufactured by JSRCorporation), 22.0 parts by weight of zinc diacrylate, 5 parts by weightof zinc oxide, an appropriate amount of barium sulfate, 0.5 parts byweight of diphenyl disulfide, and 0.9 parts by weight of dicumylperoxide. This rubber composition A was placed into a mold includingupper and lower mold halves each having a hemispherical cavity, andheated at 160° C. for 20 minutes to obtain a core with a diameter of39.8 mm. The amount of barium sulfate was adjusted such that the weightof a golf ball was appropriate.

A resin composition a was obtained by kneading 47 parts by weight of anionomer resin (the aforementioned “Himilan 1555”), 46 parts by weight ofanother ionomer resin (the aforementioned “Himilan 1557”), 7 parts byweight of a styrene block-containing thermoplastic elastomer (theaforementioned “RABALON T3221C”), 4 parts by weight of titanium dioxide,and 0.2 parts by weight of a light stabilizer (trade name “JF-90”,manufactured by Johoku Chemical Co., Ltd.) with a twin-screw kneadingextruder. The core was placed into a final mold that includes upper andlower mold halves each having a hemispherical cavity. The final mold hasa large number of pimples on the cavity face thereof. By injectionmolding, the melted resin composition a was injected around the core toform a cover with a thickness T2 of 1.45 mm. Dimples having a shape thatis the inverted shape of the pimples were formed on the cover.

A clear paint including a two-component curing type polyurethane as abase material was applied to this cover to obtain a golf ball of Example1 with a diameter of about 42.7 mm and a weight of about 45.6 g.

Examples 2 to 11 and Comparative Examples 1 to 3

Golf balls of Examples 2 to 11 and Comparative Examples 1 to 3 wereobtained in the same manner as Example 1, except the specifications ofthe core and the cover were as shown in Tables 4 to 6 below. Thespecifications of the core are shown in detail in Tables 1 and 2 below.The specifications of the cover are shown in detail in Table 3 below.

[Flight Test]

A driver (trade name “XXIO8”, manufactured by DUNLOP SPORTS CO. LTD.,shaft hardness: R, loft angle: 10.5°) was attached to a swing machinemanufactured by Golf Laboratories, Inc. A golf ball was hit under acondition of a head speed of 40 m/sec, and the ball initial speed, thespin rate, and the flight distance were measured. The flight distance isthe distance between the point at the hit and the point at which thegolf ball stopped. The average value of values obtained by 12measurements is shown in Tables 4 to 6 below.

[Feel at Impact]

Twenty golf players hit golf balls with putters and were asked aboutfeeling. The evaluation was categorized as follows on the basis of thenumber of golf players who answered, “the feeling was favorable”.

A: 16 to 20

B: 10 to 15

C: 3 to 9

D: 0 to 2

The results are shown in Tables 4 to 6 below.

TABLE 1 Specifications of Core (parts by weight) A B C D Polybutadiene100 100 100 100 Zinc oxide 5 5 5 5 Zinc acrylate 22.0 24.8 21.6 21.2Barium sulfate * * * * Diphenyl disulfide 0.5 0.5 0.5 0.5 Dicumylperoxide 0.9 0.9 0.9 0.9 2-naphthalenethiol — — — — Crosslinking 160 160160 160 temperature (° C.) Crosslinking time 20 20 20 20 period (min) *Appropriate amount

TABLE 2 Specifications of Core (parts by weight) E F G H Polybutadiene100 100 100 100 Zinc oxide 5 5 5 5 Zinc acrylate 20.8 26.0 21.0 27.0Barium sulfate * * * * Diphenyl disulfide 0.5 0.5 0.5 0.5 Dicumylperoxide 0.9 0.9 0.9 0.8 2-naphthalenethiol — — — 0.1 Crosslinking 160160 140 160 temperature (° C.) Crosslinking time 20 20 20 20 period(min) * Appropriate amount

TABLE 3 Specifications of Cover (parts by weight) a b c d Himilan 155547 — — Himilan AM7329 — 40 40 50 Himilan 1557 46 — — — Himilan 1605 — 5257 47 RABALON T3221C 7 8 3 3 Titanium dioxide 4 4 4 4 JF-90 0.2 0.2 0.20.2 H2 (Shore D) 57 59 61 63

TABLE 4 Results of Evaluation Ex. 1 Ex. 2 Ex. 3 Ex. 4 Ex. 5 CoreComposition A B C D E Diameter (mm) 39.8 39.8 39.8 39.8 39.8 Df1 (mm)4.85 4.15 4.95 5.05 5.15 H1o (shore C) 58 62 57 56 55 H1s (shore C) 7276 71 70 69 H1s − H1o 14 14 14 14 14 NF(2)1 (Hz) 1355 1631 1315 12751235 Cover Composition a a b c d T2 (mm) 1.45 1.45 1.45 1.45 1.45 H2(Shore D) 57 57 59 61 63 Ball NF(2)2 (Hz) 1855 2031 1865 1875 1885 Df2(mm) 4.25 3.65 4.25 4.25 4.25 V (Hz) 952 944 988 1025 1062 Feel atimpact A B A B C Flight test Spin (rpm) 2500 2588 2450 2400 2350 Initialspeed (m/s) 57.50 57.70 57.63 57.76 57.89 Flight distance (m) 195.9196.0 196.9 198.0 199.0

TABLE 5 Results of Evaluation Ex. 6 Ex. 7 Ex. 8 Ex. 9 Ex. 10 CoreComposition A B F C G Diameter (mm) 39.5 39.5 39.5 39.5 40.2 Df1 (mm)4.85 4.15 3.85 4.95 4.75 H1o (shore C) 58 62 64 57 60 H1s (shore C) 7276 78 71 68 H1s − H1o 14 14 14 14 8 NF(2)1 (Hz) 1347 1623 1761 1307 1316Cover Composition a a a b a T2 (mm) 1.60 1.60 1.60 1.60 1.25 H2 (ShoreD) 57 57 57 59 57 Ball NF(2)2 (Hz) 1927 2103 2191 1937 1816 Df2 (mm)4.23 3.63 3.38 4.23 4.27 V (Hz) 1029 1021 1017 1066 939 Feel at impact AB C B A Flight test Spin (rpm) 2520 2608 2683 2470 2650 Initial speed(m/s) 57.52 57.72 57.82 57.65 57.48 Flight distance (m) 195.8 195.9195.7 196.8 194.4

TABLE 6 Results of Evaluation Comp. Comp. Comp. Ex. 11 Ex. 1 Ex. 2 Ex. 3Core Composition G D A H Diameter (mm) 39.5 39.5 39.1 39.5 Df1 (mm) 4.755.05 4.85 4.00 H1o (shore C) 60 56 58 58 H1s (shore C) 68 70 72 78 H1s −H1o 8 14 14 20 NF(2)1 (Hz) 1304 1267 1507 1779 Cover Composition a c a aT2 (mm) 1.60 1.60 1.80 1.60 H2 (Shore D) 57 61 57 57 Ball NF(2)2 (Hz)1834 1947 2167 2269 Df2 (mm) 4.23 4.23 4.21 3.66 V (Hz) 965 1102 11621083 Feel at impact A D D D Flight test Spin (rpm) 2685 2420 2540 2618Initial speed (m/s) 57.52 57.78 57.54 58.29 Flight distance (m) 194.3197.9 195.7 198.4

As shown in Tables 4 to 6, the golf ball of each Example has excellentfeel at impact upon putting and has excellent flight performance upon ashot with a driver. From the results of evaluation, advantages of thepresent invention are clear.

The golf ball according to the present invention is suitable for, forexample, playing golf on golf courses and practicing at driving ranges.The above descriptions are merely illustrative examples, and variousmodifications can be made without departing from the principles of thepresent invention.

What is claimed is:
 1. A two-piece golf ball including a core and acover positioned outside the core, wherein a value V calculated by thefollowing mathematical formula is equal to or less than 1080 Hz,V=NF(2)2−2/3*NF(2)1, wherein NF(2)1 represents a secondary naturalfrequency of the core, and NF(2)2 represents a secondary naturalfrequency of the golf ball.
 2. The two-piece golf ball according toclaim 1, wherein a difference (H1s−H1o) between a Shore C hardness H1sat a surface of the core and a Shore C hardness H1o at a central pointof the core is equal to or greater than
 10. 3. The two-piece golf ballaccording to claim 1, wherein the value V is equal to or less than 1040Hz.
 4. The two-piece golf ball according to claim 3, wherein the value Vis equal to or less than 1000 Hz.
 5. The two-piece golf ball accordingto claim 1, wherein an amount of compressive deformation Df1 of the coreis equal to or greater than 4.1 mm.
 6. The two-piece golf ball accordingto claim 1, wherein an amount of compressive deformation Df2 of thetwo-piece golf ball is equal to or greater than 3.5 mm.
 7. The two-piecegolf ball according to claim 1, wherein a thickness T2 of the cover isequal to or greater than 0.80 mm but equal to or less than 2.00 mm. 8.The two-piece golf ball according to claim 1, wherein a Shore D hardnessH2 of the cover is equal to or greater than 50 but equal to or less than65.